Light from multiple colored LEDs (e.g. red, green, blue) of a light emitting unit may be mixed to obtain white or colored light.
However, the output (e.g. emitted flux and spectral distribution) of the LEDs will vary due to for instance (differential) ageing and temperature influence, binning, etc. For instance, when creating white light by mixing red, green and blue LEDs, the temperature effects will already result in a visible color difference after a small rise in temperature.
Hereto, a number of models for achieving a stable output (color) of such a light emitting unit have been proposed, for instance temperature feed forward (TFF), color coordinates feedback (CCFB), flux feedback (FFB), or a combination of flux feedback and temperature feed forward (FFB+TFF). In temperature feed forward, a temperature sensor is used for detecting the current temperature of the LEDs (e.g. by measuring the temperature of a common heat sink whereto the LEDs are mounted). By knowing how the LED output varies with temperature, the current temperature can be used to adjust the output of the LEDs to achieve a desired light output of the light emitting unit. In color coordinates feedback, the current LED colors are measured. In flux feedback, the current flux of each LED or LED color is measured. The current flux (feedback) is then compared to a desired flux or output, whereby the output of the LEDs can be adjusted accordingly to achieve a desired light output of the light emitting unit. In flux feedback, a single optical sensor is preferably used to detect the output of the LEDs or LED colors. To this end, for detecting the output of each LED color, the sensor is time-multiplexed over all LED colors. This means that the LED colors are switched on/off in a sequential manner, and the instantaneous output is determined for all switched-on LEDs for each measurement. An exemplary pattern for one frame or period T is illustrated in FIG. 1a. For measurement m1, all LEDs are switched off, and the background light is measured. For measurement m2, the blue LEDs is switched on, and the blue output may be determined by subtracting m1 from m2. For measurement m3, also the green LED is switched on, and the green output may be determined by subtracting m2 from m3. Finally, for measurement m4, also the red LED is switched on, and the red output may be determined by subtracting m3 from m4. In this way, the current output of each LED color may be determined with a single sensor.
The above flux feedback solution (optionally in combination with temperature feed forward) will work fairly well in an environment with constant stray background light falling onto the sensor. Namely, the above model assumes that the background light detected at m1 remains the same for the other measurements m2-m4. However, in some practical cases, the background light is not constant. Imagine for example that another nearby LED light emitting unit (or other pulsed light source or a CRT screen) operates with a frequency which precisely coincides and consequently interferes with measurement m4. That is, the other LED light emitting unit is on at m4, but may be off at the other measurements. This non-constant interference would significantly affect the above model and reduce the color and brightness accuracy.
In an attempt to solve this problem, the international patent application publication no. WO 2006/054230 A1 discloses a control system and method for controlling the light output of a light emitting unit having at least one LED and emitting light of at least one color, which system and method basically uses an output feedback model. In one embodiment, the LEDs of the light emitting units are energized a-periodically, as illustrated in FIG. 1b. By energizing the LEDs a-periodically, the likelihood of significantly reducing interference cased by a periodically pulsed external light source is said to be high.
However, the space (i.e. the frame) within which the LED emission windows can be varied is limited, which means that a pulsed external light source with similar or lower frequency as the light emission unit still may cause interference.